Solutions of lithium aluminium hydride

ABSTRACT

The invention relates to a solution of lithium aluminum hydride in 2-methyltetrahydrofuran or a solvent mixture containing 2-methyltetrahydrofuran, a method for producing said solution and use of the same.

The present invention provides a solution of lithium aluminium hydride, a process for the preparation of this solution and the use thereof.

Lithium aluminium hydride (LiAlH₄) is a strong reducing agent which dissolves well in polar solvents such as diethyl ether (Et₂O) or tetrahydrofuran (THF). In the solid, powdered state it is difficult to handle on account of its high reactivity towards air and water, as well as the risk of dust explosions. On an industrial scale, therefore, LiAlH₄ solutions in organic solvents are preferably used. LiAlH₄ can be prepared by reacting an aluminium halide, in particular aluminium chloride (AlCl₃), with lithium hydride (LiH) in an ethereal solvent. In the synthesis developed by Schlesinger et al. (J. Amer. Chem. Soc. 69, 1199 (1947)), Et₂O is employed as the solvent:

LiAlH₄ is very readily soluble in Et₂O with a proportion of 28%, but this solvent requires complex safety measures owing to its low flash point and boiling point. It is therefore avoided as far as possible in industrial applications.

Commercially supplied LiAlH₄ solutions generally contain no Et₂O but are supplied as a 10% solution in THF or a 15% solution in THF/toluene (Chemetall brochure “Industrial Use”). THF has a significantly higher boiling point than Et₂O. Thus, the boiling point of THF is 66° C. but that of Et₂O is only 35° C.

There are various methods of preparing solutions of LiAlH₄ in THF or solvent mixtures containing THF. One possibility consists in the reaction of sodium aluminium hydride with lithium chloride (LiCl) (WO-A-97/06097):

Disadvantages of this process are the relatively long reaction times and poor yields.

On an industrial scale, THF solutions of LiAlH₄ are prepared by dissolving solid, solvent-free LiAlH₄ in THF or THF/co-solvent. In this case, the solid LiAlH₄ is first synthesised as a solution in diethyl ether, which is subsequently evaporated completely at elevated temperatures and under reduced pressure. However, this process also has serious disadvantages. Firstly, it is relatively expensive owing to its complexity.

Furthermore, the thermal loading of the LiAlH₄ during evaporation leads to partial decomposition as in diagram 3:

Li₃AlH₆ is insoluble and therefore cannot be used for reduction processes. The solutions prepared by dissolving solid LiAlH₄ are markedly cloudy owing to the content of elemental aluminium and Li₃AlH₆; the solids content must be removed, e.g. by filtration or decantation. This is because residual contents of elemental aluminium not only impair the handling properties but also cause a clear deterioration in the storage stability, since they catalytically accelerate the decomposition process according to diagram 3.

In order to achieve a solution of LiAlH₄ in THF, the synthesis according to diagram 1 can also, in principle, be carried out directly in THF. However, this has the disadvantage that, on the one hand, the solubility of the AlCl₃ employed is only relatively low. Thus, at room temperature (RT), the solubility of AlCl₃ in THF is only 16%, whereas in Et₂O it is 57%. On the other hand, the LiCl formed as a by-product is readily soluble in THF.

Thus, the solubility of LiCl in THF is 4.8%, but in Et₂O it is <0.001%. In this way, only dilute solutions of LiAlH₄ in THF could be prepared, and these are at the same time contaminated with LiCl.

The object of the present invention is to overcome the disadvantages of the prior art.

According to the invention, the object is achieved by the features of the main claim. Preferably embodiments are found in the subclaims.

In particular, the present invention is intended to provide a LiAlH₄ solution which is as concentrated as possible, safe to handle and obtainable inexpensively.

According to the invention, these objects are achieved by a solution of LiAlH₄ in 2-methyltetrahydrofuran (2-MeTHF) or a solvent mixture containing 2-methyltetrahydrofuran. Preferably according to the invention, the preparation of LiAlH₄ is also carried out in 2-MeTHF or a solvent mixture containing 2-MeTHF.

Surprisingly, it has been found that 2-MeTHF dissolves AlCl₃ very well but LiCl hardly at all:

TABLE 1 RT solubility (wt. %) of AlCl₃ LiCl LiAlH₄ In diethyl ether 57 <0.001 28 In THF 16 4.8 21 In 2-MeTHF 45 0.3 17

Thus, the necessary conditions exist for preparing concentrated LiAlH₄ solutions:

The process according to the invention for the preparation of the LiAlH₄ solution according to the invention is generally carried out as described below, without limiting the invention thereto:

Powdered LiH is suspended in 2-MeTHF or a hydrocarbon or a 2-MeTHF/hydrocarbon mixture, and a solution of AlCl₃ in 2-MeTHF or a 2-MeTHF/hydrocarbon mixture is added to this suspension. The particle size of the LiH is preferably <30 μm. The reaction is strongly exothermic and the heat of reaction is dissipated by external cooling.

The reaction temperature is only limited in the lower range by the solidification temperature and in the upper range by the boiling point of the solvent or solvent mixture employed. In practice, the reaction is carried out at temperatures of between 0° C. and 110° C. At higher temperatures, product decomposition can be expected to start. The preferred reaction temperature is between 20 and 90° C. LiH is generally used in excess, preferably with a 5 to 20% excess.

After complete conversion, the LiCl formed as a by-product is separated off. This operation can take place according to the prior art by a solid/liquid separation step, e.g. by filtration, centrifugation or decantation. Work-up by filtration is preferred. The filtrates are generally clear or only a little cloudy, colourless or slightly yellow. In particular, they are not contaminated by elemental aluminium. Should the concentration of LiAlH₄ be too low, they can be concentrated by evaporation, preferably in vacuo.

Furthermore, it has surprisingly been found that the solubility of LiCl in the LiAlH₄ solution according to the invention exceeds the solubility in dilute LiAlH₄ solutions known from the pure binary system. This can be seen from the following table:

TABLE 2 Molar ratio LiAlH₄ conc. 2-MeTHF: LiCl solubility (wt. %) LiAlH₄ (wt. %) (mole %) Notes 0 n.a. 0.3 n.a. only 2-MeTHF 5 8.3 1.2 22 only 2-MeTHF 8 5.1 1.6 16 only 2-MeTHF 12 3.0 0.9 7 only 2-MeTHF 11.4 2.2 0.14 1.1 2-MeTHF/ toluene 14 1.6 0.08 0.6 2-MeTHF/ toluene n.a. = not applicable

It is clear that, as the 2-MeTHF:LiAlH₄ ratio falls, the LiCl solubility decreases markedly. To achieve the lowest possible concentration of LiCl, a 2-MeTHF:LiAlH₄ ratio of 3.0 should preferably not be exceeded in the synthesis mixture, and particularly preferably one of 2.2. This can be achieved either by using the highest possible concentrations of AlCl₃ solutions in pure 2-MeTHF or by employing solvent mixtures. Surprisingly, it has been found that aluminium chloride also dissolves well in mixtures of 2-MeTHF and a hydrocarbon. Thus, for example, the solubility of AlCl₃ in a mixture of 76% 2-MeTHF and 24% toluene is about 45%. In this solution, there is a 2-MeTHF:AlCl₃ molar ratio of 1.5.

Instead of, or in a mixture with, toluene, other hydrocarbons can also be used, preferably aromatic hydrocarbons such as ethylbenzene, xylenes, cumene, or aliphatics such as cyclohexane, hexane, methylcyclohexane, heptane, individually or as a mixture of at least two of these solvents.

If it is desired to prepare a LiAlH₄ solution with the lowest possible LiCl content, the synthesis is preferably carried out in a mixture of 2-MeTHF and a hydrocarbon, particularly preferably a 2-MeTHF/toluene mixture. In a mixture of this type, the 2-MeTHF:AlCl₃ molar ratio should not fall below a value of 1.3.

In a particularly preferred embodiment of the process of the invention, LiH is suspended in a hydrocarbon, e.g. toluene, and a solution of AlCl₃ in a 2-MeTHF/hydrocarbon mixture, preferably in a 2-MeTHF/toluene mixture, is added. In this solution, the 2-MeTHF:AlCl₃ ratio is between 1.3 and 3.0, with a 2-MeTHF:AlCl₃ ratio of 1.5 to 2.0 being preferred.

In comparison with the water-miscible THF, which has a marked tendency to form peroxide, 2-MeTHF offers substantial advantages when used in organic synthesis. On the one hand, owing to its higher boiling point, it allows higher temperatures to be set without the need to increase the external pressure. Thus, the boiling point of 2-MeTHF is 78° C. compared with only 66° C. for THF. This is an advantage for the hydrogenation of low-reactivity functional groups such as e.g. organic chlorides.

Surprisingly, it has also been found that LiAlH₄ solutions containing 2-MeTHF exhibit particular thermal stability: in contrast to solutions in THF, they decompose endothermically. Even at relatively high temperatures, therefore, no so-called “runaway” scenario can be expected. This makes it possible to work safely even at high temperatures.

The thermal stability of LiAlH₄ solutions containing 2-MeTHF is illustrated in FIG. 1. This compares the results of Differential Scanning Calometry tests in a Radex apparatus on a 12% solution of LiAlH₄ in 2-MeTHF/toluene with that of a 15% solution of LiAlH₄ in THF/toluene. In each case, 2 g were weighed out and the rate of heating was 45 K/h.

Curve 1: 12% solution of LiAlH₄ in 2-MeTHF/toluene: ΔT; Curve 2: 12% solution of LiAlH₄ in 2-MeTHF/toluene: pressure; Curve 3: 15% solution of LiAlH₄ in THF/toluene: ΔT; Curve 4: 15% solution of LiAlH₄ in THF/toluene: pressure.

The user draws further advantages from the fact that 2-MeTHF and water are miscible only to a limited extent. Thus, the solubility of 2-MeTHF in water is 15.1% but the solubility of water in 2-MeTHF is only 5.3%. It is therefore possible to isolate an organic synthesis product together with 2-MeTHF by phase separation. Only relatively minor product and solvent losses can be expected through the aqueous phase, from which the solvent cannot generally be recovered. For this reason, 2-MeTHF is simple to recycle.

The invention provides in detail:

-   -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the LiAlH₄ content is at least 11 wt.         %;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the LiAlH₄ content is at least 14 wt.         %;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the LiAlH₄ concentration is at least 5         wt. % and the molar ratio of 2-MeTHF:LiAlH₄ does not exceed the         value of 3.0;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the LiAlH₄ concentration is at least 5         wt. % and the molar ratio of 2-MeTHF:LiAlH₄ does not exceed the         value of 2.2;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the LiAlH₄ concentration is at least         10 wt. % and the molar ratio of 2-MeTHF:LiAlH₄ does not exceed         the value of 2.2;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the solvent contains at least one         hydrocarbon;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the hydrocarbon is an aromatic         hydrocarbon;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the aromatic hydrocarbon is selected         from toluene, ethylbenzene, xylene or cumene or a mixture of at         least two of these hydrocarbons;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the aromatic hydrocarbon is toluene;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the hydrocarbon is an aliphatic         hydrocarbon;     -   a solution of LiAlH₄ in 2-MeTHF or a solvent mixture which         contains 2-MeTHF, wherein the aliphatic hydrocarbon is selected         from cyclohexane, hexane, methylcyclohexane or heptane or a         mixture of at least two of these hydrocarbons;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein AlCl₃ is reacted with LiH in 2-MeTHF         or a solvent containing 2-MeTHF;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein the LiH concentration in the feed is         between 3 and 17 wt. %, preferably between 5 and 15 wt. %;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein the LiH powder employed has a particle         size of <30 μm;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein the reaction temperature is between 0         and 100° C., preferably between 20 and 90° C., particularly         preferably between 30 and 80° C.;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein the reaction is carried out in the         presence of hydrocarbons;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein the hydrocarbons are selected from         toluene, ethylbenzene, xylene, cumene, cyclohexane, hexane,         methylcyclohexane, heptane or a mixture of at least two of these         hydrocarbons;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein the hydrocarbon is toluene;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein the molar ratio of 2-MeTHF:AlCl₃ is         between 1 and 10, preferably between 1.3 and 5.0;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein the LiCl forming as a by-product is         separated off;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein, to remove LiCl, the solution is         concentrated to a molar ratio of 2-MeTHF:LiAlH₄ of no more than         3.5 and the precipitated LiCl is separated off by a solid/liquid         separation step, preferably by filtration;     -   a process for the preparation of the LiAlH₄ solution according         to the invention, wherein, to remove LiCl, the reaction solution         is concentrated in vacuo, preferably under a pressure of between         1 and 500 mbar, and at a temperature of between 20 and 100° C.;     -   the use of the LiAlH₄ solution according to the invention as a         reducing agent.

The invention is explained below by examples, without being limited thereto:

EXAMPLE 1 Preparation of an 8% LiAlH₄ Solution in Pure 2-MeTHF

In an inerted, i.e. dried and filled with protective gas, e.g. nitrogen or argon, 1-litre reactor having a double-walled jacket, 41.7 g LiH powder are suspended in 40 g 2-MeTHF. The suspension is heated to 35° C. and a solution of 158.7 g AlCl₃ in 370 g 2-MeTHF is added dropwise within 3 hours, while stirring well. After a 1.5-hour post-reaction period at 35 to 40° C., the reaction suspension is emptied on to a G3 glass filter.

407 g of a slightly yellow, clear solution are obtained. Analysis (mmol/g) Li Al H⁻ Cl⁻ 2.50 2.13 9.33 0.37 From this, the following is calculated: LiAlH₄ = 8.1 wt. % (based on Al analysis) LiCl = 1.6 wt. % ({circumflex over (=)}17 mole %, based on Al) Yield: 73% of theory.

The filter residue is washed with 2-MeTHF; a further 16% of theory is obtained in the form of the wash filtrates.

EXAMPLE 2 Preparation of a low LiCl Content LiAlH₄ Solution in 2-MeTHF by an Evaporation Process

117 g of the LiCl-containing wash filtrates from example 1 are concentrated by distillation at 40 to 60° C. and under a reduced pressure of 250 mbar. A total of 80.0 g solvent is distilled off. After removing approximately ⅔ of the quantity mentioned, colourless crystals begin to precipitate.

On completion of the distillation process, the suspension is cooled to room temperature and filtered through a G3 fritted glass filter until clear.

Final weight: 36.0 g

The analysis gave the following results:

TABLE 3 Li Al H⁻ Cl⁻ LiAlH₄ LiCl Substance sample (mmol/g) (wt. %) (mole %) Before distillation 0.77 0.45 1.87 0.18 1.7 40 After distillation 2.11 1.75 7.03 0.28 6.6 16

By means of a further evaporation step to a LiAlH₄ concentration of 12.3%, the content of soluble LiCl is reduced to 7 mole %.

EXAMPLE 3 Preparation of an 11% LiAlH₄ Solution in 2-MeTHF/toluene

In an inerted 1-litre reactor, 59.9 g LiH powder are suspended in 140 g toluene and heated to 75° C. Into this suspension, 504 g of a 44% solution of AlCl₃ in a mixture of 2-MeTHF and toluene and in a 2MeTHF:AlCl₃ ratio of 1.5 are metered within 80 minutes. On completion of the addition, stirring is continued for a further 30 minutes at 80° C. and the suspension is then emptied on to a preheated filter.

253 g of a yellow solution with the following composition are obtained: Analysis (mmol/g) Li Al H⁻ Cl⁻ 3.02 3.00 12.4 0.034

The LiAlH₄ concentration is 11.4% ({circumflex over (=)}98% of theory) and the LiCl concentration is 1.1 mole %, based on LiAlH₄.

The filter residue is washed twice with toluene. In all the filtrates, 17.4 g LiAlH₄ content is obtained ({circumflex over (=)}91% of theory). 

1-21. (canceled)
 22. A solution comprising LiAlH₄ in a solvent, wherein the solvent comprises 2-MeTHF.
 23. The solution according to claim 22, wherein solvent further comprises at least one hydrocarbon.
 24. The solution according to claim 22, wherein the LiAlH₄ content is at least 11 wt. %.
 25. The solution according to claim 22, wherein the LiAlH₄ content is at least 14 wt. %.
 26. The solution according to claim 22, wherein the LiAlH₄ concentration is at least 5 wt. % and the molar ratio of 2-MeTHF:LiAlH₄ does not exceed the value of 3.0.
 27. The solution according to claim 22, wherein the LiAlH₄ concentration is at least 10 wt. % and the molar ratio of 2-MeTHF:LiAlH₄ does not exceed the value of 3.0.
 28. The solution according to claim 23, wherein the hydrocarbon comprises at least one aromatic hydrocarbon.
 29. The solution according to claim 28, wherein the at least one hydrocarbon is selected form the group consisting of toluene, ethylbenzene, xylene and cumene.
 30. The solution according to claim 23, wherein the hydrocarbon is toluene.
 31. The solution according to claim 23, wherein the hydrocarbon is at least one an aliphatic hydrocarbon.
 32. The solution according to claim 23, wherein the hydrocarbon comprises at least one of cyclohexane, hexane, methylcyclohexane or heptane.
 33. A process comprising preparing a solution according to claim 22 by reacting AlCl₃ with LiH in the solvent comprising 2-MeTHF.
 34. The process according to claim 33, wherein the solvent further comprises at least one hydrocarbon.
 35. The process according to claim 34, wherein the at least one hydrocarbon is selected from the group consisting of toluene, ethylbenzene, xylene, cumene, cyclohexane, hexane, methylcyclohexane and heptane.
 36. The process according to claim 33, wherein the solvent further comprises toluene.
 37. The process according to claim 33, wherein the concentration of LiH in a feed is between 3 and 17 wt. %.
 38. The process according to claim 33, wherein the molar ratio of 2-MeTHF:AlCl₃ is between 1 and
 10. 39. The process according to claim 33, wherein the LiH is added as a powder and has a particle size of <30 μm.
 40. The process according to claim 33, wherein the reaction temperature is between 0 and 100° C.
 41. The process according to claim 33, wherein the reaction temperature is between 20 and 90° C.
 42. The process according to claim 33, wherein the reaction temperature is between 30 and 80° C.
 42. The process according to claim 33, further comprising the step of separating any LiCl forms as a by-product from the solution.
 44. The process according to claim 33, further comprising the step of concentrating the solution to a molar ratio of 2-MeTHF:LiAlH₄ of no more than 3.5 and separating any precipitated LiCl by a solid/liquid separation step.
 45. The process according to claim 44, wherein the solid/liquid separation step is filtration.
 46. The process according to claim 33, wherein the reaction solution is concentrated in vacuo.
 47. The process of claim 46, wherein the in vacuo pressure is between 1 and 500 mbar and the temperature is between 20 and 100° C.
 48. A process comprising chemically reducing a reducible chemical by adding a sufficient amount of a the solution of claim 22 to act as a reducing agent to reduce the chemical. 